A stable Drosophila X chromosome can be destabilized by association for only one generation with the unstable X chromosome (Uc) or its derivative. Occasionally, male flies carrying the destabilized X chromosome transmit a mutant allele not apparent in their phenotype (cryptic mutation). We are interested in learning about the origin and the underlying molecular mechanisms for transmission of cryptic mutations. In our system, two X-linked loci exhibit a high frequency of generation and transmission of cryptic mutation. These are cut wings (ct) and forked bristles (f) loci. In a particular cross, for example, more than 2% of males carrying the destabilized X chromosomes produce at least one sperm with cryptic ct. Our data show that a transposable element called gypsy is involved in transmission of the mutations. A number of separate lines of evidence indicate that the ordinary meiotic recombination process is not involved in generating the cryptic mutations. Suppressible insertion mutations, transposon-mediated instability, heteroduplex structure of cryptic mutant loci, or involvement of extrachromosomal replicating elements are some of the models we have been testing.